It's the day before the Greens launch an election manifesto tilted heavily towards economic issues and Caroline Lucas, the party leader, wants to talk to potential constituents in Brighton about jobs.

There's a problem. The somewhat Stalinist outlines of the concrete Jobcentre Plus she is trying to visit have clearly rubbed off on the security guards: they don't care that the event was arranged weeks ago – she can't come in.

It's an irritation, but it is clearly difficult to dampen the mood of a woman who, in three weeks' time, is heavily tipped to win her party's first ever seat at Westminster.

"It would be absolutely fantastic," she says, taking advantage of the unexpected break in her schedule to sit in a nearby coffee bar. "We've waited for this moment and worked for this moment for so long. And anyway, I can go and see the Jobcentre on 7 May."

Lucas will give up the European parliament seat she holds if the bookmakers are correct and she takes Brighton Pavilion on 6 May.

She is the favourite for a variety of reasons. Her party has a strong presence on the local council: with 13 representatives, the same as Labour, it is the joint opposition, while the constituency overall has a predominance of bicycling, recycling bohemians and students.

The party is working flat out in Brighton, one of three key target seats along with Lewisham, in south London, and Norwich South. In the latter seat, Lucas's deputy, 28-year-old Adrian Ramsay, may remove Charles Clarke, the former home secretary.

In the two-and-a-half years since she was adopted as a candidate, Lucas has split her time between attending Brussels and campaigning on the south coast, seeking in particular to expand support into the more deprived districts further from the seafront.

Adding further assistance to her cause, David Lepper, the sitting Labour MP, is standing down after seeing a once-strong majority eroded since 1997. The Conservatives are now viewed as the main threat.

Most of all, the Greens might win because voters finally believe that they have a chance.

Lucas talks of a tipping point. "It's massively important for people to know that we can win," she says. "We find on the doorsteps that so many people want to vote Green. They just want to know that a Green vote has a chance of getting someone elected."

Many new supporters are disillusioned Labour voters, attracted less by Lucas's promises on environmental matters than on her party's plans to increase investment, boost jobs and introduce a "citizens' pension" of £170 a week.

The manifesto, officially launched in London tomorrow, has economic goals as its opening chapter. The environment comes fourth.

"The economy is at the forefront of the campaign because we believe people don't know us so well for these issues," Lucas says. "It's not that we suddenly think the environment is less important."

On issues such as equality and social justice Labour has "vacated the space" and former supporters are moving to the Greens, she argues. "They've been holding their nose for so long, they're on the point of expiring. Now they think it's safe to take the clothes peg off and they feel quite excited that there is this party which stands for the values they believe in. It's like a homecoming, it's almost quite moving."

The coffee drunk, it's back on the campaign trial, but on more familiar turf. Lucas is cutting the ribbon – a green one, of course – to give a formal opening to a new shop offering ethically made clothes, natural beauty products and health treatments. She chats happily with staff and customers, a pre-midday glass of champagne thrust into her hand but left untouched.

The surrounding tangle of narrow lanes, lined with coffee shops and fashionable bars, would seem natural Green territory, but even here there is reticence.

Claire Willis, a 33-year-old teacher, stopping for a glance as she pushes her two-week-old daughter, Nancy, past the shop, confesses that while she voted Green in council elections, she will plump for Labour next month.

"It's not so much holding my nose as crossing my fingers," she says. "I know it sounds silly, but I want to go for a party that could make a real difference in parliament. The Greens are just too small. Basically, I'll do anything to keep the Tories out."

Standing nearby smoking a cigarette is a more hopeful specimen for Lucas. As an avowedly left-wing NHS worker, David Allerton should be a natural Labour man.

"You'd expect me to be a core Labour supporter, but they're like quasi-Tories these days," the 42-year-old says. "I couldn't bring myself to vote for anyone in 2005, but I'm very tempted to go Green this time."

In many ways it is a perfect political storm for the Greens, with the expenses scandal spreading voter disenchantment over all the major parties. Lucas agrees: "The conditions are very good yes. Voters are disillusioned with the other parties. People are coming to us with positive reasons, but there's much more of an interest in looking for an alternative. And we are a real alternative."

Lucas's ambitions do not stop with Brighton. Once inside Westminster, she believes, the Greens could expand into a mainstream political force, particularly if the electoral system is reformed.

So could she envisage a Green administration in the coming decades? "Absolutely – and not too many decades ahead. Don't forget, it was only around 24 years between the first Labour MP getting elected and the first Labour government."

Guardian specialists dissect the Green party manifesto and give their views on the most important passages. Click on the highlighted paragraphs to read their comments or use the annotations bar at the top of the page to flick through them